Production of foam boards of high compressive strength from styrene polymers

ABSTRACT

Process for the production of foam boards having a minimum thickness of 20 mm and a cross-sectional area of at least 50 cm 2  by extruding a mixture of the styrene polymer, from 1 to 10% by weight, based on the styrene polymer, of a blowing agent, wherein the blowing agent contains at least one ether from the group consisting of dimethyl ether, methyl ethyl ether and methyl vinyl ether.

The present invention relates to a process for the production of foamboards of high compressive strength by extruding a mixture of styrenepolymer, a blowing agent or blowing agent mixture and, if desired,conventional additives and/or auxiliaries.

A multiplicity of blowing agents has been proposed for the production offoam boards based on styrene polymers by extrusion.

For example, EP-A-464 581 discloses a process for the production of foamboards having high compressive strength and a cross-sectional area of atleast 50 cm², in which a mixture of styrene polymer, from 5 to 16% byweight, based on the styrene polymer, of a blowing agent, and, ifdesired, conventional additives is extruded, where the blowing agentused is a mixture comprising

a) from 3 to 70% by weight of an alcohol or ketone having a boilingpoint of from 56 to 100° C.,

b) from 5 to 50% by weight of carbon dioxide and c) from 0 to 90% byweight of a saturated C₃ - to C₅ -hydrocarbon and/or ahydrogen-containing chlorofluorocarbon or fluorocarbon having a boilingpoint of from -30° C. to +30° C.

The foam boards produced by known processes have the disadvantage thatthe final values, for example for heat distortion resistance, becomeestablished relatively slowly.

JP-A 60/145 835 discloses the production of a laminate board containinga polystyrene foam layer, in which the polystyrene foam layer has beenproduced by extrusion using an ether, such as dimethyl ether, methylethyl ether or methyl vinyl ether, as blowing agent.

The example describes the production of a board with a thickness of 6 mmusing 3.7 parts by weight of dimethyl ether, based on polystyrene, asblowing agent.

It has also been disclosed that adequate dimensional stability andcompressive strength of foam boards having a thickness of 20 mm or more,in particular of ≧50 mm, is frequently not guaranteed if blowing agentswhich diffuse rapidly out of the foam are used.

Furthermore, the use of blowing agents of high blowing force frequentlygives foams having a high content of open cells.

Thus, in the process of EP-A-464 581, the production of high-qualityfoam boards having a thickness of 20 mm or more is frequently onlypossible in the presence of chlorofluorocarbons or fluorocarbons asblowing agent.

It is an object of the present invention to provide a process for theproduction of foam boards from styrene polymers having a minimumthickness of 20 mm, in which no toxic blowing agents are used and whichgives foams containing closed cells which have only low water-absorptioncapacity and, in addition to good dimensional stability and compressivestrength, have, in particular, high heat distortion resistance. It is afurther object of the present invention to provide a process whichenables the rapid establishment of the final values, in particular forheat distortion resistance, in the foam boards produced.

We have found that this object is achieved by a process for theproduction of foam boards having a minimum thickness of 20 mm and across-sectional area of at least 50 cm² by extruding a mixture of astyrene polymer, from 1 to 10% by weight, based on the styrene polymer,of a blowing agent and, if desired, conventional additives and/orauxiliaries, in which the blowing agent used is at least one ether fromthe group consisting of dimethyl ether, methyl ethyl ether and methylvinyl ether.

For the purposes of the present invention, styrene polymers are taken tomean polystyrene and copolymers of styrene containing at least 50% byweight of copolymerized styrene.

Examples of suitable comonomers are α-methylstyrene, ring-halogenatedstyrenes, ring-alkylated styrenes, acrylonitrile, esters of(meth)acrylic acid with alcohols having 1 to 8 carbon atoms, N-vinylcompounds, such as N-vinylcarbazole, maleic anhydride or alternativelysmall amounts of compounds containing two polymerizable double bonds,such as butadiene, divinylbenzene and butanediol diacrylate.

According to the invention, the blowing agent or blowing agent mixtureis used in an amount of from 1 to 10% by weight, preferably from 3 to 9%by weight, in particular from 4 to 8% by weight, based on the styrenepolymer. It contains at least one ether from the group consisting ofdimethyl ether, methyl ethyl ether and methyl vinyl ether, with thecontent of the ethers in the blowing agent or blowing agent mixturepreferably does not exceed 10% by weight, particularly preferably 30% byweight, in each case based on the blowing agent or blowing agentmixture. Ether contents of less than 0.5% by weight, based on theblowing agent or blowing agent mixture, are generally ineffective.

Particularly favorable according to the invention are ether-containingblowing agent mixtures containing up to 60% by weight, preferably from 5to 55% by weight, of carbon dioxide and up to 70% by weight of asaturated C₃ -C₅ -hydrocarbon, fluorocarbon, chlorofluorocarbon and/oran alcohol or ketone having a boiling point of from 56 to 100° C.

Examples of suitable blowing agent constituents are propane, n-butane,i-butane, neopentane, n-pentane, i-pentane, 1,1,1,2-tetrafluoroethane,1,1-difluoroethane, 1,1-difluoro-1-chloroethane, ethanol, methanol andacetone. Ethanol is preferred.

In a preferred embodiment, the blowing agent used is a mixturecomprising

a) from 30 to 100% by weight of dimethyl ether,

b) from 0 to 60% by weight of carbon dioxide and

c) from 0 to 30% by weight of ethanol.

In a particularly preferred embodiment of the invention, the blowingagent used is a mixture comprising

a) from 45 to 95% by weight of dimethyl ether and

b) from 5 to 55% by weight of carbon dioxide.

Conventional additives which can be added to the styrene polymer/blowingagent mixture are antistatics, stabilizers, dyes, lubricants, fillers,flameproofing agents and/or nucleating agents in conventional amounts.

The foam boards are produced according to the invention in a mannerknown per se by extrusion. The styrene polymer plasticated by warming ismixed intimately with the blowing agent mixture and the additives in anextruder. The mixture then passes through a calming zone, in which it iscooled to from about 100° to 120° C. with constant stirring and issubsequently extruded through a die to give boards.

According to the invention, the foam boards have a minimum thickness of20 mm, preferably 30 mm, particularly preferably 50 mm. Thecross-sectional area is at least 50 cm², preferably at least from 100 toabout 1000 cm². The invention permits the production of boards having athickness of up to 200 mm, a width of up to 1200 mm and a density offrom 25 to 70 g/l.

In a preferred embodiment of the novel process, foam boards having athickness of from 20 to 100 mm are produced using a blowing agentmixture comprising

a) from 10 to 90% by weight, preferably from 20 to 70% by weight, of atleast one ether from the group consisting of dimethyl ether, methylethyl ether and methyl vinyl ether,

b) from 10 to 60% by weight, preferably from 30 to 55% by weight, ofcarbon dioxide and

c) from 0 to 40% by weight, preferably from 1 to 20% by weight, of asaturated C₃ -C₅ -hydrocarbon and/or alcohol having a boiling point offrom 56° to 100° C.

In a preferred embodiment of the novel process, foam boards having athickness of at least 100 mm are produced using a blowing agent mixturecomprising

a) from 40 to 100% by weight, preferably from 60 to 100% by weight, ofat least one ether from the group consisting of dimethyl ether, methylethyl ether and methyl vinyl ether,

b) from 0 to 30% by weight, preferably from 5 to 25% by weight, ofcarbon dioxide and

c) from 0 to 30% by weight, preferably from 1 to 15% by weight, of asaturated C₃ -C₅ -hydrocarbon and/or alcohol having a boiling point offrom 56° to 100° C.

The invention has numerous advantages. The novel process allows thesimple and economical production of thick foam boards from which theblowing agent escapes rapidly without reducing the compressive strength.The foam boards produced by the novel process achieve the final valuesfor compressive strength and heat distortion resistance after only ashort storage time.

It is possible to use environmentally friendly blowing agents which arealready present in nature or, like dimethyl ether, are broken down veryrapidly. Dimethyl ether is broken down in air with a half life of a fewdays and thus does not accumulate in the atmosphere.

The invention is described with reference to Examples 1 to 8. In theExamples, parts are by weight.

EXAMPLES 1 and 2

100 parts of polystyrene having a melt flow index of 4.5, 0.35 part ofhexabromocyclododecane as flameproofing agent, 0.14 part of dicumylperoxide and 0.51 part of talc for regulating the cell size were fedcontinuously to an extruder having an internal screw diameter of 53 mm.At the same time, a blowing agent mixture having the composition shownin Table 1 was injected continuously into the extruder through an inletopening in the extruder. The gel which had been uniformly compounded inthe extruder was then, after a residence time of 20 minutes, cooled toan exit temperature of 119° C. and extruded into the atmosphere througha die measuring 50×1.5 mm. The foam was passed through a mold channelconnected to the extruder, giving a foamed polystyrene board having across-section of 150 mm×60 mm.

EXAMPLES 3 to 8

100 parts of polystyrene having a melt flow index of 4.5 and the amountsshown in Table 2 of hexabromocyclododecane (HBCD), dicumyl peroxide andtalc were fed continuously to an extruder having an internal screwdiameter of 120 mm. At the same time, the blowing agent mixture havingthe composition shown in the Table was injected continuously into theextruder through an inlet opening in the extruder. The gel which hadbeen uniformly compounded in the extruder was then, after a residencetime of 40 minutes, extruded into the atmosphere through a die with awidth of 300 mm. The foam was passed through a mold channel connected tothe extruder, giving a foamed polystyrene board having a width of 650mm. The die gap and the thickness and density, the latter determined inaccordance with DIN 53420, of the resultant foam boards are shown inTable 2.

Each of Examples 1 to 8 gave a uniform, closed-cell and shape-stablefoam board.

                  TABLE 1                                                         ______________________________________                                             Amount of                                                                     blowing agent,                                                                % by weight                  Den-  Board-                                     based on    CO.sub.2 CH.sub.3 OCH.sub.3                                                                    sity  thickness                             Ex.  polystyrene % by wt. % by wt.                                                                              g/l   mm                                    ______________________________________                                        1    5.8         --       100     55    60                                    2    7.0         43.0     57.0    63    60                                    ______________________________________                                    

For Examples 5 and 6, the dimensional stability of the polystyreneboards was determined in accordance with DIN 53431. In each case,storage was for 67 days. The results are shown in Table 3.

                                      TABLE 2                                     __________________________________________________________________________       Amount of blowing                                                             agent                         Dicumyl       Board                                                                              Foam                         % by wt., based                                                                         C.sub.2 H.sub.5 OH                                                                 CH.sub.3 OCH.sub.3                                                                  CO.sub.2                                                                           HBCD                                                                              peroxide                                                                           Talc                                                                              Die gap                                                                            thickness                                                                          density                   Ex.                                                                              on polystyrene                                                                          % by wt.                                                                           % by wt.                                                                            % by wt.                                                                           Parts                                                                             Parts                                                                              Parts                                                                             mm   mm   g/l                       __________________________________________________________________________    3  6.7       --   100   --   1.00                                                                              0.21 0.64                                                                              1.5  60   31                        4  7.7       15.6 84.4  --   1.00                                                                              0.20 0.85                                                                              2.1  100  31                        5  7.0       15.2 62.9  21.9 0.95                                                                              0.19 0.15                                                                              2.1  80   30.9                      6  7.1       14.7 63.6  21.7 0.95                                                                              0.19 0.15                                                                              1.6  70   31.1                      7  6.9       --   100   --   1.00                                                                              0.20 0.15                                                                              >3.5 200  30.5                      8  7.0       --   100   --   1.00                                                                              0.20 0.29                                                                              >3.5 200  33.1                      __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________    Dimensional stability at 85° C. in %                                                       Dimensional stability at 90° C. in %               Ex.                                                                              Length                                                                             Width                                                                              Thickness                                                                            Length                                                                             Width                                                                              Thickness                                       __________________________________________________________________________    5  -0.73                                                                              -0.64                                                                              -0.48  -2.01                                                                              -1.94                                                                              -1.67                                           6  -0.67                                                                              -0.68                                                                              -0.66  -1.98                                                                              -2.01                                                                              -1.70                                           __________________________________________________________________________

We claim:
 1. A process for the production of foam boards having aminimum thickness of 60 mm and a cross-sectional area of at least 50 cm²by extruding a mixture of a styrene polymer, from 1 to 10% by weight,based on the styrene polymer, of a blowing agent and, conventionaladditives and/or auxiliaries, wherein the blowing agent used is amixture comprising from 5 to 55% by weight of carbon dioxide and atleast 30% by weight of an ether from the group consisting of dimethylether, methyl ethyl ether and methyl vinyl ether, no chlorofluorocarbonor fluorocarbon being present.
 2. A process as claimed in claim 1,wherein the ether used is dimethyl ether.
 3. A process as claimed inclaim 1, wherein the foam boards having a minimum thickness of 100 mmare produced using a blowing agent comprising at least 40% by weight ofone of said ethers.
 4. A process as claimed in claim 1, wherein up to30% by weight of ethanol is present in the blowing agent mixture.
 5. Aprocess as claimed in claim 1, wherein the blowing agent used is amixture comprisinga) from 45 to 95% by weight of dimethyl ether and b)from 5 to 55% by weight of carbon dioxide.